The rapidly fading afterglow from the gamma-ray burst of 1999 May 6

We report on the discovery of the radio afterglow from the gamma-ray burst of 1999 May 6 (GRB 990506) using the Very Large Array. The radio afterglow was detected at early times (Delta t = 1.5 days) but began to fade rapidly sometime between 1 and 5 days after the burst. If we attribute the radio em ission to the forward shock from an expanding fireball, then this rapid ons et of the decay in the radio predicts that the corresponding optical transi ent began to decay between 1 and 5 minutes after the burst. This could expl ain why no optical transient for GRB 990506 was detected in spite of numero us searches. The cause of the unusually rapid onset of the decay for the af terglow is probably the result of an isotropically energetic fireball expan ding into a low-density circumburst environment. At the location of the rad io afterglow, we find a faint (R similar to 24 mag) host galaxy with a doub le morphology.